Electric machines may be utilized in motor vehicles for converting mechanical energy into electrical energy. These types of separately excited electric machines may be designed as claw pole generators, for example. Such electric machines may generate a multiphase three-phase current. Electric machines may include a rotor with an excitation winding (rotor winding) and a stator with a multiphase stator winding. With the aid of a rectifier connected downstream from the stator winding, a multiphase three-phase current may be rectified into a direct current and supplied, for example, to a direct voltage electrical system of the motor vehicle.
In hybrid vehicles, for example, electric machines may also be operated in motor mode and used as the drive of the hybrid vehicle. The electric machine may assist an internal combustion engine of the motor vehicle, in particular at low rotational speeds at which this internal combustion engine does not yet deliver its full torque (boost mode, turbo lag compensation).
The electric machine may be regulated in both operating modes with the aid of an excitation current through the excitation winding, or with the aid of a phase current through the stator windings, as control variables.
Utilizing the electric machine in particular as a drive in a vehicle may require frequent switching on and switching off of the electric machine. Switching the electric machine on and off, in particular at high rotational speeds, may prove to be problematic. If the phase voltage is switched on first, this may result in excessively high overshooting of the phase current. If an excitation voltage is switched on first, a synchronous generated internal voltage is initially induced in the stator which may interact unfavorably with the phase voltage to be subsequently switched on, which may result in high electrical and mechanical loads on the electric machine.
It is therefore desirable to provide an option for optimizing the switching on and switching off of an electric machine of a motor vehicle in order to avoid undesirable current spikes and associated electrical and mechanical loads.